Change semantics of validate_cart hook

src/pretix/presale/views/checkout.py

@@ -5,6 +5,8 @@ from django.utils.translation import ugettext_lazy as _
 from django.views.generic import View
 
 from pretix.base.models import CartPosition
+from pretix.base.services.cart import CartError
+from pretix.base.signals import validate_cart
 from pretix.multidomain.urlreverse import eventreverse
 from pretix.presale.checkoutflow import get_checkout_flow
 
@@ -12,12 +14,20 @@ from pretix.presale.checkoutflow import get_checkout_flow
 class CheckoutView(View):
     def dispatch(self, request, *args, **kwargs):
         self.request = request
-        has_cart = CartPosition.objects.filter(
-            cart_id=self.request.session.session_key, event=self.request.event).exists()
-        if not has_cart and "async_id" not in request.GET:
+        cart_pos = CartPosition.objects.filter(
+            cart_id=self.request.session.session_key, event=self.request.event
+        )
+
+        if not cart_pos.exists() and "async_id" not in request.GET:
             messages.error(request, _("Your cart is empty"))
             return redirect(eventreverse(self.request.event, 'presale:event.index'))
 
+        try:
+            validate_cart.send(sender=self.request.event, positions=cart_pos)
+        except CartError as e:
+            messages.error(request, str(e))
+            return redirect(eventreverse(self.request.event, 'presale:event.index'))
+
         flow = get_checkout_flow(self.request.event)
         for step in flow:
             if not step.is_applicable(request):